It's sometimes advantageous to use cash on the French side, as some establishments offer 1:1 Euro:Dollar for payment in cash. Now that the Euro has fallen, that's no longer a huge advantage.

Check the terms on all of your credit cards, as more are jumping on the no fee bandwagon; you may have one and not realize it. Discover Card, while not as widely accepted on the island as Master Card and Visa, is one that also charges no fee.

A while ago someone mentioned a gas station that takes credit cards, but I'm afraid I don't remember which one that was. Our experience has always been cash only at gas stations.